Coherent-trapped helical mode in parity-time symmetric metamaterials

Coaxial optical subwavelength elements support helical modes Lm with different topological indexes m. Here we propose to couple the two bright L_±1 modes with the dark one L₀ via a parity-time (PT) symmetric perturbation. We show that the cascading coupled configuration is similar to a three-level atomic system, and supports a special hybridized mode Lc via a classic analog of coherent-population-trapping effect. Resonant frequency of Lc is independent of the PT-symmetric perturbation. Populations in L_±1 can be manipulated by tuning the PT-symmetric perturbation, and no population is trapped in L₀. Since the L_±1 modes are associated with optical waves of opposite circular polarizations, the polarization of transmitted wave is independent of the polarization of incidence but solely determined by the PT-symmetric perturbation. Such an effect can be utilized to manipulate the polarization state of light. Numerical simulation in a well-designed coaxial metamaterial verifies our analysis.